I'm just throwing this out there as a spitball, but how wide does that foil bubble wrap stuff come? For an extreme situation, one could use it as a tarp to drape over the roof and down the sides. The best defense is not to let the sun hit your RV surface or through the glass in the first place. Reflective bubble wrap on the inside of a window does work, but it would be more effective to stop the heat before it gets on the inside of the glass. Sure, gonna take some doing to roll up a sheet that big and to store it. And it's gonna look pretty odd. And certainly would draw the ire or the uppity blue haired Nazis at some of the private campgrounds. But if it's that hellish hot out.....